What is the average volume space of the human body?

The human body has a density close to that of water ~ 1000 kg/m^3
an average person weight maybe 70 kg, which gives an approximate body volume of

V=m/M = 70/1000 = 0,07 m^3

Although, a human body density could definitely change depending on anything that could be implanted into the body, and since fat can float (think of soap) it should alter the values accordingly.